Insurers and pharmacy benefit managers, which act as intermediaries between employers and drug companies, are also keeping an eye on employers.

“You have this influx of new threats,” said Paul Keckley, a health policy analyst.

Two big health insurers, Aetna and Cigna, recently merged with pharmacy benefit managers. Anthem said Wednesday that it was moving quickly to set up an in-house operation, terminating its relationship with Express Scripts, a pharmacy benefit manager, earlier than planned after Cigna acquired it.

Until recently, large employers had generally not been viewed as competitors to the insurers and pharmacy benefit managers that handle their employees’ claims, although the announcement of the ABJ venture expressed frustration with the United States health care system.

The Big Three’s new venture “doesn’t have a name yet, much less any revenues,” said David Johnson, the chief executive of 4sight Health, a consulting firm. “It strikes me that they should be trying to court ABJ rather than sticking them in the eye.”

The real threat to Optum may be Amazon, which has already made some forays into the pharmacy business and is willing to sacrifice profits as it builds market share. Optum is vulnerable because its customers — particularly employers — have little insight into its businesses, which range from health data services to pharmacy benefits, said Michael Turpin, a former UnitedHealth executive who is an executive vice president at USI, an insurance brokerage.

“It is truly the most opaque of all black boxes in health care,” Mr. Turpin said. He added, “Amazon is lying in the grass and will someday pop up and do something disruptive around pharmacy.”

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In its filings, Optum assumes the worst, arguing that the new venture will eventually decide to offer services to the wider world. It describes Mr. Smith as asking “the court to ignore reality and believe that ABC is just a nonprofit start-up with the sole goal of improving health outcomes for the employees of its founding companies.”